A tandem-type image forming apparatus forms a color composite toner image on a transfer body by superimposing toner images of respective colors one above the other. More specifically, after a first toner image formed on an image carrier is transferred to the transfer body, a second toner image formed on the image carrier is transferred from the image carrier onto the transfer body having the first toner image that is previously transferred, such that the second toner image is superimposed on the first toner image over its entire area.
Due to the resistivity of toner of the first toner image, a transfer current tends to flow through a section other than the section at which the second toner image is superimposed over the first toner image. Accordingly, the section at which the second toner image is superimposed on the first toner image is not provided with a sufficient level of transfer current, thus lowering a toner transfer rate. The toner transfer rate refers to the amount of second toner image that is actually superimposed on the first toner image with respect to the total amount of second toner image formed on the image carrier. The lowered transfer rate causes a decrease in image density as the amount of toner that is transferred to the transfer body decreases, which is undesirable.
Japanese Patent Application Publication No. 2003-186284 discloses an image forming apparatus, which obtains a ratio indicating the degree of overlap between the second toner image and the first toner image, and determines a transfer current based on the obtained ratio.
While the technique disclosed in Japanese Patent Application Publication No. 2003-186284 should be able to suppress degradation in image quality as the transfer rate increases, a portion of the first toner image that is not superimposed by the second toner image may be transferred from the transfer body back to the image carrier. This phenomenon, called reverse transfer, reduces the amount of toner in the first toner image, thus causing degradation in image quality.
If the image forming apparatus implements the cleaner-less method as described in any one of Japanese Patent Application Publication Nos. H05-313431, H08-54771, and 2005-338636, toner having a specific color that is reverse-transferred to the image carrier may be collected by a developing device to be mixed with toner of the other color. Unless a toner bottle is replaced with a new one, the color reproducibility of the image forming apparatus stays at a low level due to mixing of toner of different colors.